Tuning system



J. M. MILLER TUNING SYSTEM Aug. 18, 1931.

Filed July 3, 1950 2 Sheets-Sheet l l l l 70c 800 900 I000 000 SIGNAL AT 550 Aug. 18, 1931. J, L Q 1,819,299

TUNING SYSTEM Filed July 3, 1930 2 Sheets-Sheet 2 Patented Aug. 18, 1931 UNITED STATES PATENT OFFICE JOHN M. MILLER, OF PHILADELPHIA, PENNSYLVANIA, ASSIGNOR TO ATWATER KENT MANUFACTURING COMPANY, OF PHILADELPHIA, PENNSYLVANIA, A CORPORATION OF PENNSYLVANIA Application filed July 3,

llfy invention relates to tuning systems utilizable partlcularly in tunlng radio receiving apparatus, and more particularly in tuning radio receiving apparatus of the 4 super-heterodynetype.

In accordance with my invention, a circuit or system is tunable to oscillations or currents of any frequency within a range, while largely suppressing or eliminating the cffeet of undesired current or oscillations having a frequency difiering therefrom, especially to a predetermined or fixed extent, by recourse to inductanceand a single Variable tuning capacity, which latter with a portion of the inductance effects a series resonant path for the current of undesired frequency, which accordingly has insubstantial eifect upon or is bypassed from any circuit, vacuum tube or device whose terminals are those of the series resonance combination.

Further in accordance with my invention, the inductance of the series resonance combination is wholly or in part shunted by a capacity, which may be of fixed magnitude,

for suitably modifying the effect of the inductance in the series resonance combination at diiferent parts of a frequency range, and more particularly in the high frequency portion of a given frequency range.

Further in accordance with my invention a system or circuit of the character aforesaid may precede or follow another tunable circuit in cascade therewith. A plurality of J tunable circuits or systems of the character aforesaid may be disposed in cascade, constituting the entire orany suitable portion of a cascade of tunable circuits.

Further in accordance with my invention,

the circuit or circuits of the character aforesaid is or are untilizedin a superheterodyne system, preferably in advance of the first detector tube, and preferably in immediate association with the input electrodes of the first detector tube, the series resonance combination being tunable to frequencies differing from the frequency of the desired current or oscillations by an amount substantially double the intermediate frequency of the superheterodyne system, for materially reducing or eliminating interference, of the TUNING SYSTEM 1930. Serial No. 465,603.

character commonly known as image or double spot interference, otherwise occurring.

Further in accordance with my invention, the circuit or circuits of the character aforesaid may be used in single dial or uni-control systems, in which variable tuning elements in different circuits or paths are operated mechanically in unison; and more particularly in uni-control superhetrodyne systems in which there is included in the uni-control a variable reactance or frequency-determining element of a generator of local oscillations whose frequency is varied as the tuning of the circuit or circuits progresses to produce a beat .of the aforesaid intermediate frequency.

My invention resides in a tuning system and in features of arrangement and proportionment of the character hereinafter described and claimed.

For an understanding of my invention, and for an illustration of some of the various forms it may take, reference is to be had to the accompanying drawings in which:

Fig. 1 is a digrammatic view of a. tuning system embodying my invention.

Fig. 2 is a graph comprising characteristic curves illustrative of my invention.

Fig. 3 is a diagrammatic view of a modified tuning system in accordance with my invention.

Fig. 4 is a digrammatic View of cascaded tuned circuits including a tuning system or circuit in accordance with my invention.

Fig. 5 is a diagrammatic view of a superheterodyne receiving system embodying my invention.

Referring to Fig. 1, L and L1 are inductanees together associated with the variable condenser C for tuning tothe desired oscillations or currents of various frequencies impressed upon the circuit in any suitable way,

i for example through a primary P coupled difference of potential exists across the terminals of the condenser C, and to the terminals of the condenser C are generally connected the input terminals of a vacuum tube, amplifier, detector, or of any other suitable device. In the present instance, however, the terminals employed are those indicated 'at 1 and 2, which in the example illustrated are connected respectively to the grid 9 and the cathode c of an ampllfying, detecting or other suitable vacuum tube, either of the usual three electrode type, or of a multi-grid type, including those of the so-called shield or screen grid type.

The arrangement is such that in effect the maximum potential at resonance is not impressed upon the grid and cathode, but a lesser potential is impressed thereon due to the fact that the grid 9 is in effect connected to a point intermediate the terminals of the inductance comprising L and L1 in series. Nevertheless, this lesser potential is at a maximum when the arallel resonance com bination L1, L and is tuned to resonance for the desired current or oscillations representing the signal or message to be received.

The combination LG, however, is tunable to series resonance at a frequency higher than that ofcthe oscillations or current representing the desired signal, and higher frequency, the combination LC is of extremely low impedance, thereby substantially eliminating the effect of the current of other and higher frequency upon the input elements g and a. At series resonance of the combination LC, substantially the only impedance is the resistance of the winding of the inductance L, and this is so inconsequential thatthe aforesaid efi'ect across the grid g and cathode c is negligible.

While oridinarily the selectivity of the parallel resonance combination Llplus LG is suflicient to exclude oscillations or cur-" rents of other frequencies, there are cases where, as in superheterodyne systems hereinafter described, it is desirable or necessary to take further steps to eliminate such other frequencies; anda circuit of the character of Fig. 1 is provided with very powerful or effective means for procuring such additional discrimination, which in the example described is effected by a single variable tuning condenser available for both tuning to the desired signal frequency and to efiect a series resonance path for the undesired signalfre uency.

or the desired signal frequency the magvnitude of the inductance L and L1 and the magnitude of the capacity C are such as to effect resonance. The connection of one of the terminals of the translating device,'as

the grid 9 of the vacuum tube, to a point between the inductance elements L1 and L roduces some loss in resonant voltage, for

it is somewhat less than the resonant voltage and as to such other across the condenser C, but nevertheless there will be a sufficiently large difierence of-potential between the points 1 and 2 to procure effective transfer to the device Whose terminals are connected to the points 1 and 2. This loss in voltage, however, is readily made up by amplification, by one or more of the vacuum tubes of the receiving system. For any setting of the tuning condenser C for desired frequency, the series combination LO exhibits practicall no impedance or reactance for some ot er and higher frequency whose effect, therefore, upon a device connected to the points 1 and 2 is practically eliminated. A single value of the, inductance L will not maintain perfect series resonance for the interfering frequency, when difi'ering a fixed amount from the desired frequency, and when the condenser G is varied throughout its range of tuning to the desired signals. Nevertheless, there is substantial improvement sired signal frequencies in the discrimination against the inlerfering signal, with perfect results, however, at only one such different and higher interfering frequency.

Speaking in terms of present broadcast frequencies, which range from 550 k. 0. (kilocycles) to 1500 k. c., and referring to Fig. 2, curve A is a curve or plot of magnitudes of inductance L in microhenries against frequencies in kilocycles. Curve A shows the variation of magnitude L necessary for a particular broadcast receiver ,which shall discriminate against a higher undesired frequency differing by, say, 260 k. c. from the desired frequency, throughout the range of frequencies in which interference is possible,

throughout the range of de- Curve A indicates the necessity for variationof themagnitude of the inductance L, particularly for certain conditions hereinafter described, and to avoid actual variation of the magnitude of the inductance L, but to cause it satisfactorily to operate with suflicient exactitude, there is associated therewith another element, which will now be described.

In IN? 3 the arrangement is the same as that of ig. 1,- except that in shunt to the whole or a suitable part of the component L of the entire inductance is shunted a condenser Co of suitable capacity, which, as diagrammatically represented, includes also the istributed capaclty of the winding of the inductance L. This arrangement, I have found, produces a marked improvement in the elimination of the effect of the undesired signal frequency. The parallel combination of L and Co has an effective inductance, which increases with increasing frequency, which is in the desired direction, particularly for the purposes hereinafter described in connection with superheterodyne receivers. This parallel combination does not effect exact variation of the effect of inductance L, as required by curve A of Fi 2, but affords a suitable compromise satis actory from a practical standpoint. The parallel combination LCo does, however, afford exact magnitudes of the effect of the inductance L for two desired frequencies in the range, and in any event very much enhances the discrimination 'in general throughout the frequency range of the desired signals. This is indicated by dotted curve B, Fig. 2, where the approximation to the exact requirements indicated by curve A is shown to be close through the high frequency portion of the range, and is exact at any two suitable frequencies as those corresponding with the points 3 and 4, and between these frequencies, corresponding to the points 6 and 4-the approximation is close enough for practical purposes, as indeed is the case even tothe left of the point 3 for lower frequencies, because in that portion of the entire range the selectivity of a circuit, such as indicated in Figs. 1 and 3. is greater in any event than for the higher frequencies of the range, and because for the higher frequencies of the range the percentage difference between the desired signal and interfering signal is less.

For the pur )oses described, the parallel combination L 0 is so adjusted or proportioned as to procure suitable or proper values of the effective inductance in the higher frequency portion of the range. The maladjustment, such as indicated by the divergence of the curves A and Bis extremely small in the high frequency portion of the range.- The curve B corresponds with an inductance L of about 110 microhenries and capacity C0 of about 26 micro-microfarads'; the inductance L1 may be of any other suitable magniture, as, for example, substantially equal .to L, but in any event the sums of the inductances L and L1 are those essential for tuning with the Variable capacity C to the desired si nal frequencies.

eferring to Fig. 4, D is a receiving antenna or equivalent, between which and earth E,

or other counter capacity or equipotential conductor, is connected the primary p of a radio frequency transformer, any suitable amount ofwhich may be included in the antenna path by the switch a. Coupled to the primary 1) is the secondary s, which with the primary P is tunable to the desired signal .frequeaicy by the variable condenser GIL The primary P is coupled to the inductance L1 of a circuit of the character of Fig. 3, havin as before, the inductance L shunted by the capacity Co, in series with the condenser C. This circuit intervenes between the first-named circuit sPCl and a following tuned circuit comprising the inductance L2, shunted by the variable condenser C2, for to the desired signal frequency. The three tuned circuits are in cascade, each tunable to the desired hequency. The circuit L2, C2 is coupled to the intermediate circuit of the character of Fig. 3 through a small coupling condenser K.

Across the terminals 4 and 5 are connected the input terminals of an amplifying, detecting or other vacuum tube, or the terminals of any other translating device. Fig. 4 illustrates that a circuit of the character of either Fig. 1 or Fig. 3 may comprise one of a series of cascaded tunable circuits, and that it may either precede or follow a tunable circuit of the cascade. I

As indicated by dotted lines in Fig. 4, the rotors of the several tuning condensers C, C1 and C2 may be mechanically coupled to effect a uni-control or single dial operation. This system may precede any type of apparatus, particularly radio frequency amplifying apparatus; and particularly in a su- 'results a beat current having a frequency equal to the difference between the frequencies of locally generated and desired signal oscillations. The superimposed oscillations are impressed upon the input terminals of a detector, generally a Vacuum tube detector, in whose plate circuit appear oscillations of the aforesaid beat frequency, which latter is also termed the intermediate frequency, for it is still a high frequency intermediate the frequency of the desired signal oscillationsand the audio frequency currents representing the desired signal, as speech, music, etc. The oscillations of intermediate or beat frequency are amplified by the so-called intermediate frequencyamplifier, which is usually designed to amplify only a narrow band of frequencies. \Vhen,,as in broadcast re ception, it is desired to receive signal oscillations of various frequencies, corresponding to different broadcasting stations, the frequency of the locally produced and superimposed oscillations is adjusted, so that the difference or beat frequency is substantially constant or falls within the band to which the intermediate. frequency amplifier is generally timed. Undesired signal oscillations, those of frequencies other than that of the desired signal, produce in general beat frequencies falling outside 'of the band to which the intermediate frequency amplifier is tuned, and the undesired signal oscillations are in effect not received, or in any event produce negligible effect, and the system is accordingly selective.

There is a condition, however, under which undesired signals can produce serious interference, because, it is evident, one signal frequency above and another signal frequency to an equal extent below the frequency of the locally generated oscillations will produce the same specified difference or intermediate frequency, and consequently be converted by the detector to the same intermediate frequency, and accordingly there will arise serious confusion or interference between the signals of the two frequencies, when-that of only one of the frequencies is desired. Such interfering signals, producing the same intermediate frequency, are spaced apart in frequency by twice the intermediate frequency. This type of interference is frequently referred to as the double spot or image effect; and it is one of the principal objects of my present invention materially to reduce or eliminate the possibility of such interference.

This object is attained by recourse to a circuit of the character of Fig. 1, and preferably of the character of Fig. 3, utilized in advance of the first detector of a superheterodyne system, and preferably immediately in advance thereof, as indicated in Fig. 5,

where the first detector is the vacuum tube V, in this instance a screen grid detector having the control grid 9, cathode 0 heated by the electric heater 72., screen or shield grid 1), and anode or plate d. The grid 9 and cathode 0 are connected as indicated in Fig. 3, with the intervention, between the cathode 0 and the terminal 2 of the preceding tunable circuit, of the coil or transformer winding 81 in series with the resistance r, the latter shunted by capacity is. To the coil s1 is coupled a coil p1 of the local oscillator O, which produces continuous oscillations of suitable amplitude, and of a frequency, determinable by the variable capacity C3, whose rotor, as indicated by the dashed lines, may be mechanically coupled for operation in unison with the rotors of the tuning condensers C 1 and C1. The local oscillator O is of the as by sound waves for transmission of music and speech. The beat current or current of intermediate frequency, appearing in the anode circuit of the detector tube V, is maintained constant by suitable adjustment of the condenser G3 with relation to the adjustment of the condenser C, and of condenser 01 if an additional tuned circuit is utilized. This intermediate frequency may be assumed to be 130 k. 'c. It is impressed upon the grid or input circuit of the first intermediate vacuum tube amplifier V1 in whose'plate, circuit is an inductance L3 tuned by capacity comprisinga condenser K3 for a narrow band of intermediate frequencies, within which lies the aforesaid frequency of 130 k. c., for example The amplified intermediate frequency cuirent is then impressed upon the input circuit of a generally similar or equivalent vacuum tube amplifier V2 in whose output circuit is a similar combination of inductance L4 with tuning capacity comprising a condenser K4 for tuning to the same narrow band of intermediate frequencies. The intermediate frequency thus further amplified is impressed upon the grid circuit of the second detector comprising the vacuum tube V3, inwhose anode circuit appears the audio frequency current representing the .slgnal, as speech, music, or the like. Coupled to the plate circuit of'the second detector V3 is the input circuit of an audio frequency amplifier tube V4 whose output circuit is coupled to the input circuit of the push-pull amplifier tubes V5 and V6, in whose output system is disposed the signal translating instrument or loud speaker I.

A system of the character described may utilize any type of power, such as direct or alternating current, as well understood in the art. In the example illustrated, the power supply is of the alternating current type. The power supplyconductors 6 and 7 connect through any suitable means asa switch or lamp socket with a source of commercial alternating current, from which current is accordingly delivered to the primary of the transformer T, having a secondary b delivering alternating current of suitably high potential, rectified by a thermionic tube V7, for supplying through a suitable filter as indicated uni-directional or substantially direct current to the anode circuits of the several tubes V to V6 inclusive and 0. One or more additional secondaries S1 of transformer T supply alternating current, as ,for heating the cathode heater resistances h of the several tubes, all as well understood in the art.

In a superheterodyne system, generally of the character above described, it has been the practice to employ cascaded circuits, in advance of the first detector tube, tuned to the desired signal fre uency, to exclude the undesired signal di ering from the desired signal by twice the intermediate frequency. i

It is evident, however, that in the case where the interfering signal is that of a nearby powerful station, while at the same time the desired signal is relatively weak, a very high degree of discrimination is required, and it s thls unusual degree of dlscrlmlnation which is effected in accordance with my inventlon ient, in uni-control or single dial operation is here that it is more desirable that there be exactitude,-because the selectivity is lower, and the per cent difference between desired and interfering signals is less at the high frequency range. However, to the left to operate the local oscillation generator 0 at -'of the point 3, and for the lower frequencies,

a frequencyhigher than that of the desired signal by the value of the intermediate frequency, in which case the interfering signal producing the aforesaid double spot or image will be higher in frequency than the desired signal by twice the value of the intermediate frequency. For such circumstances particularly, the circuit arrangement of Figs 1 and 3, and particularly the latter are unusually effective, to render negligible or insubstantial the effect of the interfering or double spot signal frequency.

In order to maintain perfect operation for the purposes stated, it is necessary to increase the inductance L as the capacity of. the variable condenser C is decreased. Curve A of Fig. 2 indicates the variation of the inductance L necessary for a particular broadcast receiver throughout the range of frequencies in which interference of the character aforesaid is possible on the partof other simultaneously operating broadcast stations. The receiver, exemplified by Fig. 2, operates with an intermediate frequency of 130 k. e., in which case a station radiating waves of a frequency of 1500.k. 0. tends to cause interference with reception of a desired signal of 1240 k. c.; and a station radiating waves at a frequency of 810 k. '0. causes interference with a desired signal frequency of 550 k. c. Accordingly, the range through- -out which interference of the type referred to is encountered lies in the receiving range between 1240 k. c. and 550 k.c. It is forthis latter range that the curve Ashows thenec; essaryvalues of inductance L to maintain the condition of series resonance of the combination LC as C is varied throughout this range for tuning to a desired signal frequency.

By the parallel combination LCo, Figs. 3

and 5, the effect of the inductance L increases with increasing frequency or with decreasing tuning settings of the condenser C; and this change is in the desired sense or direction. Exact magnitudes of effect of inductance L are procured at any two frequencies, (corresponding with points 3 and 4 for example) in the high frequency portion of the range within which the interference of the type described occurs, i. e., between 1240 and 550 k. c. Orthe two fre-- quencies at which the efiect of inductance L is exactly right may be 5-50 and1240 k. e., in which case there will be deviation from cori'cct values in the middle of the range in otherwise interfering slgnal.

the effect is, nevertheless, highly beneficial, though not perfectly attained. For a superheterodyne receiver of the character described, the magnitudes of inductance L and capacity C0 above given are satisfactory, and constitute one example of many proportions of the combination suitable for various ranges and purposes. Accordingly when varying the capacity of the condenser C, if there occurs at the same time a very powerful signal having a higher frequency and particularly one exceeding the desired signal frequency by twice the intermediate frequency, the latter will have negligible-or substantially no effect upon the detector V, and there will, therefore, be prevented the double spot type of interference in the plate circuit of the detector and beyond, with the result that in the loud speaker or signal translating instrument I there will be no, or substantiallyno, reproduction of the What I claimls:

1. A method of tuning to oscillations of potential related to the potentialdifference existing between a terminal of said loop and a point which divides one of said reactances into components one of which is in series with the other of said reactances, and varying only said other of said reactances to tune said loop to desired frequency and to bring into substantial resonance with the undesired frequency said variable reactance and said component of said one of said reactances.

2. A method of tuning to oscillations of various frequencies with additional discrimination against oscillations whose frequencies differ to a substantially fixed extent from said first-named frequencies, which comprises arr ranging inductive and capacitative reactances effecting a loop tunable to the oscillations of desired frequency, impressing upon a translating device a difference of potential related to' the potential difference existing between a terminal of said loop anda point 1 25 which divides one of said reactances into components one of which is in series with the j other of said reactances, altering the effect of said component of said one of said reactances by shunting it with reactance of the other type, and varying only said other of said reactances to tune'said loop to desired frequency and to bring into substantial resonance with the undesired frequency said variable reactance and said shunted component of said one of said reactances'.

3. In superheterodyne reception, a method of tuning to oscillations of various frequencies with additional discrimination against oscillations whose frequencies so differ from said first-named frequencies that reaction with both by locally produced oscillations produces substantially the same beat fre quency, which comprises arranging inductive and capacitative reactances effecting a loop tunable to the oscillations of a desired frequency. impressing upon a detector a potential difference dependent upon the potential difference existing between a terminal of said loop and a point which divides. one of said reactances into components one of which is in series with the other of said reactances, superimposing upon said oscillationsof desired frequency locally generated oscillations of different frequency to produce oscillations of beat frequency, and varying only said other of said reactances to tune said loop to desired frequency and to bring into substantial resonance with the undesired frequency said variable reactance and said component of said one of said reactances.

4. In superheterodyne reception, a method of tuning to oscillations of various frequencies with additional discrimination againstoscillations whose frequencies so differ from said first-named frequencies that reaction with both by locally produced oscillations produces substantially the same beat he quency, which comprises arranging inductive and capacitative rea'ctances effecting a loop tunable to the oscillations of a desired frequency, impressing upon a detector a potential dependent upon the potential diflerence existing between a terminal of said loop and a point which divides one of said reactances into components one of which is in series with the other of said reactances, superimposing upon said oscillations of desired frequency locally generated oscillations of different frequency to produce oscillations of beat frequency, altering the effect of said component of said one of said reactances by shunting it with reactance of the other type, and varying only said other of said reactances to tune said loop to desired frequency and to bring into substantial resonance with the undesired frequency said variable reactance and said shunted component.

5. A method of tuning to oscillations of various frequencies with additional discriminations against oscillations whose frequencies differ to a substantially fixed extent from said first-named frequencies, which comprises arranging inductive and capacitative reactances effecting a loop tunable to the oscillations of desired frequency, impressing upon the input terminals of a thermionic device a difference of potential dependent upon the potential difference existing between a terminal of said loop and a point which divides said inductive reactance into components one of which is in series with said capacitative reactance, and varying only said capacitative reactance to tune said loopvto desired fre-. quency and to bring into substantial resonance with the undesired frequency said variable capacitative reactance and said component of said inductive reactance.

6. A method of tuning to oscillations of various frequencies with additional discrimination against oscillations whose frequencies differ to a substantially fixed extent from said first-named frequencies, which comprises arranging inductive and capacitative reactances effecting a loop tunable to the oscillations of desired frequency, impressing upon the input terminals of a thermionic device a difference of potential dependent upon the potential difference existing between a terminal of said loop anda point which divides said inductive reactance into components one v of which is in series with said ca acitative reactance, alterln the effect of said component of saidinductive reactance by shunting it" with capacity, and varying only said variable capacitative reactance to tune-said loop to desired frequency and to bring into substantial resonance with the undesired frequencv said variable capacitative reactance and sai shunted component of said inductive reactance.

7. In superheterodyne reception, a method of tuning to oscillations-of various frequencies with additional discrimination against oscil- 'lations whose frequencies so differ from said first-naned frequencies that reaction with both by locally produced oscillations produces substantially the same beat frequency,

,7 which comprises arranging inductive and capacitatiie reactances effecting a loop tunable to the oscillations of desired frequency, impressing upon the input terminals of a" thermionic detector a potential difference dependent upon the potential diiference existing between a terminal of said loop and a point which divides said inductive reactance into components one of which is in series'with said capacitative reactance, superimposing upon said oscillations of desired frequency 10- cally generated oscillations of different frequency to produce oscillations of beat frequency, and varying only. said capacitative reactance to tune said loop to desired frequency and to bring into substantial resonance with the undesired frequency said variable capacitative reactance and said-compo nent of said inductive reactance.

8. In superheterodyne reception, a meth-V 'quencies with additional discrimination against oscillations whose frequencies so differ from said first-named frequencies that reaction with both by locally produced oscillations produces substantially the same beat fr'e' quency, which comprises arranging inductive and capacitative reactances effecting a loop tunable to the oscillations of desired frequency, impressing upon the input terminals of a thermionic detector a potential difference de.-' pendent upon the potential difference existing between a terminal of said loop and a point which divides said inductive reactance into components one of which is in series with said capacitative reactance, superimposing upon said osclllatlons of deslred frequency lo- .cally generated oscillations of different frequency to produce oscillationsof beat frequency, altering the effect of said component of said inductive reactance by shunting it ination against oscillations whose frequencies differ to a substantially fixed extent from said first namedfrequencies, comprising in ductive andv capacitative reactances, forming a loop tunable to the oscillations of desired frequency, a translating device, means for impressing thereon a difference of potential dependent upon the potential difference existing between a terminal of said loop and a point which divides one of said'reacances intocomponents one of which is in series withthe other of said reactances, and means for,

varying said 0 her of said reactances to time said loop to desired frequency and to bring into subs antial resonance with the undesired frequency said variable reactance and said component of said one of said reactances.

10. A'network for tuning to oscillations of various frequencies with additional discrimination against oscillations whose frequencies differ. to a substantially-fixed extent from said first named frequencies, comprising inductive and capacitaltive reactances forming a loop tunable'to the oscillations of desired fre 'quency, a translating device, means for impressing thereon, a difference of potential dependent upon the.p' otential difference existing between a terminal of said loop anda point which divides one of said reactan'ces into components one of which is in series'with the other of said reactanccs, a reactance of the character of said other of said reactan ces disposed in shunt with said component ofsaid one of said reactances, and meansfpr'varying said other ofsaid reactances to tune said loop to desii' ed frequency and to bring into substantial resonance with the undesired frequency said variable reactance and said shunted component of said one of said reactances.-

11. A superheterodyne receiver comprising a detector, a loop comprising inductive and capacitative reactances and tunable to the oscillations of desired frequency, said detector subjected to a potential difference dependent upon the potential difference existing betweena terminal of said loop and a point.

which divides one of said reactances int-o components one of which 1s in series with the other of sa1d reactances, a source of oscillations of'ditferent frequency, means for superimposing said last-named oscillations upon said first-named oscillations to produce oscillations of beat frequency, and means for varying said other of said reactances to tune said loop to desired frequency and to bring said variable reactance and said component of said one of said reactances into substantial resonance with undesired oscillations of a frequencywhich differs from the frequency 'of the desired oscillations by double said beat frequency.

12. A superheterodyne receiver comprising adetector, a loop comprising inductive and capacitative reactances and tunable to the oscillations of desired frequency, said detectorsubjected to a potential difference dependent upon-the potential difference exist ing between a terminal of said loop and a point which divides one of said reactances into components one of which .is in series with the other of said reactances, a source of oscillations of different frequency, means for superimposing said last-named oscillations upon said first-named oscillations to produce oscillations of beat frequency, a reactance of the character of said other of said reactances disposed in shunt with said component of said one of said reactances, and means for varying said other of said reactances to tune said loop to desired frequency and tobring said variable reactance and said shunted componentof said one of said'reactancesinto substantial resonance with undesired oscillations of a frequency which differs from the frequency of the desired oscillations by double said beat frequency.

13. A network for tuning to oscillations of various frequencies with additional discrimination against oscillations whose frequencies differ to a substantially fixed extent from said first-named frequencies,comprising induc- I potential difference existing betweena terminal of said loop and a point which divides reactance, and means for varying said capacltative reactance to tilne said loop to desired I frequency andtobring' into substantial reson ance with the undesired frequency said va- I riable capacitative reactance and said component of said inductive reactance.

14. A network for tuning to oscillations of 7 various frequencies with additional discrimination against oscillations whose frequencies differ to asubstantially fixed extent from said first-named frequencies, comprising induc-. tive and capacitative reactances forming a loop tunable to the oscillations of desired frequency, a thermionic device, means for impressing upon the input electrodes thereof a difference of potential dependent upon the potential difference existing between a terminal of said loop and a point which divides said inductive reactance into components one of which is in series with said capacitative reactance, a capacity in. shunt to said component of said inductive reactance, andmeans for varying said capacitative reactance to tune said loop to desired frequency and to bring into substantial resonance with the undesired frequency said variable capacitative reactance and said shunted component of said inductive reactance.

15. A superheterodyne receiver comprising a thermionic detector, a loop comprising inductive and capacitative reactances and tunable to the oscillations of desired frequency, said detector subjected to a potentialdifference dependent upon the potential difference existing between a terminal of said loop and a point which divides .said inductive reactance into components one of which is in series with said capacitative reactance, a source of oscillations of different frequency, means for superimposing said last-named oscillations upon said first-named oscillations to produce oscillations of beat frequency, and meansfor varying said capacitative reactance to tune said loop to desired frequency.

vand to bring said variable capacitative reactanceand said component of said inductive reactanceintosubstantial resonance with oscillationsof a frequency which differs from thefrequency of the'desired oscillations by double said beat frequency.

16. A superheterodyne receiver comprising a thermionic detector, a loop comprising inductive and capacitative reactances and Wm able to the oscillations of desired frequency,

I said detector subjected to a potential differ ence dependent upon the potential difference existing between a terminal of said loop and a point which divides said inductive reactanceinto components one of which is in series with said capacitative reactance, a source of oscillations of different frequency, means for superimposing said last-named oscillations upon said first-named oscillations to produce oscillations of beat frequency, a capacity in shunt to said component of said inductive reactance, and means for varying,

said capacitative reactance to tune said loop 370 beat frequency.

17. A superheterodyne receiver comprising a thermionic detector, 21 thermionic amplifier system associated therewith and tuned to a band of frequencies including the intermediate frequency, a local source of oscillations of variable frequency differing from the freffuencies of the desired oscillations to an ex-- tent corresponding with said intermediate frequency, and means for tuning to the desired oscillations of various frequencies with additional discrimination against undesired oscillations whose frequencies differ from said first-named frequencies by substantially double said intermediate frequency, comprising a loop including inductive and capacitative reactances and tunable to the oscillations of desired frequency, connections from the input electrodes of said detector to a terminal of said loop and to a point which divides said inductive reactance 1nto components one i of which is in series with said capacitative reactance, and'means for varying said capacitative reactance to tune said loop to desired frequency and to bring said variable capacitative reactance and said component of said inductive reactance into substantial resonance with said undesired oscillations.

18. A superheterodyne receiver comprising a thermionic detector, a thermionic amplifier system associated therewith and tuned to a band of frequencies including the intermediate frequency, a local source of oscillations of variable frequency differing from the frequencies of the desired oscillations to an extent corresponding with said intermediate frequency, and means for tuning to the desired oscillations of various frequencies with additional discrimination against undesired oscillations whose frequencies differ from said first-named frequencies by substantially double said intermediate frequency, comprising a loop including inductive and capacitative reactances and tunable to theoscillations of desired frequency, con

nections from the input electrodes of said in shunt to said component of said inductive reactance, and means for varying said capacitative reactance to tune said loop to desired mediate frequency,

frequency and to bring said variable capacitative reactance and said shunted component of said inductive reactance into substantial resonance with said undesired oscillations.

19. A superheterodyne receiver comprising a thermionic detector, a thermionic amplifier system associated therewith and tuned to a band of frequencies including the intermediate frequency, a local source of oscillations of variable frequency diifering from the frequencies of the desired oscillations to an extent corresponding with said interand means for tuning to the desired oscillations of various frequencies with additional discrimination against undesired oscillations whose frequencies difier from said first-named frequencies by substantially double said intermediate frequency, comprising a loop including inductive and capacitative reactances and tunable to the oscillations of desired frequency. con-i nections from the input electrodes of said detector to a terminal of said loop and to a point which divides said inductive react'ance into components one of which is in series with said capacitative reactance, a capacity in shunt to said component of said inductive reactance and having such magnitude that in the higher frequency portion of the frequency rangeof the desired oscillations the efiect of undesired oscillations upon said detector is a minimum, and means for vary ing said capacitative reactance to tune said 1001) to desired frequency and to bring said variable capacitative reactance and 7 said shunted component of said inductive reactance into substantial resonance with said undesired oscillations.

20. A superheterodyne receiver comprising a thermionic detector. a thermionic amplifier system associated therewith and tuned to a band of frequencies including the intermediate frequency. 2 local source of oscillations of variable frequency diifering from the frequencies of the desired os-' cillations to an extent corresponding with said intermediate frequency. and means for tuning to the desired oscillations of various frequencies with additional discrimination against received oscillations whose frequencies differ from said first-named frequencies by substantially double frequency, comprising a loop including'inductive and capacitative reactances and tunable to the oscillations of desired frequency. connections from the input electrodes of said detector to a terminal of said loop and to a point which divides said inductive reactance into components one of which is in series with said capacitative react'ance, a capacity in shunt to said component ofsaid inductive reactance and having such magnitude that at two substantially separated frequencies in the higher frequency portion of the frequency range of the desired oscillations said intermediate JOHN M. MILLER. 

